1. Field of the Invention
The present invention relates generally to a method of detecting occurrence of an overcurrent state in an electric circuit, and more particularly to an overcurrent detecting method and apparatus which are capable of determining the occurrence of an overcurrent in an electric circuit with high accuracy and reliability on the basis of sample values derived by periodically sampling an output signal of a current sensor installed in the circuit to be monitored.
2. Description of the Prior Art
Generally, in circuits such as, for example, a travel control circuit and a cargo handling circuit for a battery-powered fork lift truck, a fuse is inserted between a battery and electromechanical components such as motors and circuit elements such as power transistors and others connected together in the form of an electric circuit so that upon occurrence of an overcurrent in the circuit, the fuse is melted and breaks to thereby protect the electromechanical components as well as the electronic elements mentioned above against the overcurrent. In this conjunction, it is also known to protect electric and electronic circuit components from overcurrents with high accuracy by using a current sensor comprising a Hall element or the like and determining the occurrence of an overcurrent through comparison of sample values of the output signal of the current sensor with a preset reference current value, wherein a controllable element such as an electromagnetic contactor is automatically controlled on the basis of the result of the decision to thereby ensure that the electric motor, power transistor and others can operate within a safe operating range.
In overcurrent detecting methods known heretofore, such measures as shown in FIG. 5 have generally been adopted wherein the output current of a current sensor is periodically monitored to determine occurrence of an overcurrent state when the output current having a magnitude greater than a predetermined reference value Io continues to flow over a period exceeding a predetermined time. More specifically, referring to FIG. 5, the period or time interval T for sampling the output signal of a current sensor is so set that the period falls within a permissible duration TA of a current greater than the preset reference value Io, and the overcurrent is detected by periodically sampling the output of the current sensor at the time interval T.
With the above overcurrent detecting method, it is however impossible to detect a current Io' which is slightly smaller than the preset reference value Io and may nevertheless adversely affect the circuit components when it continues to flow over an extended time period.
This problem can certainly be solved by lowering the reference level Io to the current value Io'. However, there may occur such undesirable situations as the anti-overcurrent protective operation being performed even when the current Io' continues to flow for a period which does not adversely affect the associated circuit. As a result, performance of a machine equipped with this circuit is degraded correspondingly due to undesirable frequent activation of the overcurrent protection operation.
Under these circumstances, there exists a demand for an overcurrent detecting method and apparatus which are capable of detecting with high reliability and accuracy only such electric current which adversely affects the circuit as well as associated machine.